Aperture fluorescent lamp having uniform surface brightness

ABSTRACT

THE APERTURE OF AN APERTURE FLUORESCENT LAMP IS WIDER AT THE ENDS OF THE LAMP THAN AT THE CENTER IN ORDER TO IMPROVE THE UNIFORMITY OF THE LIGHT OUTPUT PROFILE ALONG THE ENTIRE LENGTH OF THE LAMP.

Feb. 20, 1973 T. T. SADOSKI ETAL 3,717,781

MERTUHE FLUORESCENT LAMP HAVING UNIFORM SURFACE BRIGHTNESS Filed Sept. 19, 1969 TADIUS T. SADOSKI' WALTER A. JOHNSON ATTORNEY United States Patent Inc.

Filed Sept. 19, 1969, Ser. No. 859,290 Int. Cl. H013 61/35, 61/42 US. Cl. 313-409 1 Claim ABSTRACT OF THE DISCLOSURE The aperture of an aperture fluorescent lamp is wider at the ends of the lamp than at the center in order to improve the uniformity of the light output profile along the entire length of the lamp.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the field of arc discharge lamps and particularly to the field of aperture fluorescent lamps.

Description of the prior art Tubular light sources which have a length much greater than their diameter, such as fluorescent lamps, generally project light upon a surface in such a manner that the illumination of the surface is fairly uniform in intensity, except for a gradual decrease in illumination near the ends of the illuminated plane, caused by the inherent decrease in the tubular source intensity at its ends.

In some applications, such as photocopy machine printing, a uniform distribution of illumination is necessary for uniform printing along the entire surface of the printed paper. Since all tubular light sources exhibit a fall off in light intensity near the ends of the lamp, compensating devices and lenses are needed to make the illumination more uniform from one end of the paper to the other.

'It is well known in the lamp industry that the brightness at the surface of a fluorescent lamp can be increased by scraping away the phosphor coating along a narrow strip extending the entire length of the bulb. When a reflective layer such as titanium dioxide, is interposed between the lamp bulb glass wall and the fluorescent phosphor coating, and a narrow strip of this material is scraped away, the brightness of the surface is greatly enhanced. Such a lamp is known as an aperture fluorescent lamp and is shown in US. Patent 3,225,241, issued on Dec. 21, 1965 Spencer et al.

The light output profile of such an aperture lamp also shows a decrease in light intensity at the ends thereof, similar to that of ordinary fluorescent lamps.

SUMMARY OF THE INVENTION We have found that the variation in light output profile of an aperture fluorescent lamp can be markedly reduced by making the aperture wider at the ends of the lamp than at the center. The wider aperture permits more light to radiate therethrough and increase the illumination therefrom.

Generally the wider portion of the aperture should extend only a short distance from the ends of the lamp, the extent being dependent on the output profile of the particular lamp when it has a uniform aperture along its length. Generally, also the wider aperture should not exceed about 90 of the lamp circumference since the light would then subtend over too great an arc to increase the illumination on the desired surface.

In order to prevent discoloration of the clear glass wall at the aperture from ultraviolet rays, the inside surface 3,717,781 Patented Feb. 20, 1973 of the glass should be coated with a thin protective film that is substantially opaque to ultraviolet radiation but transparent to visible light. We have found that a very thin film of ultra fine aluminum oxide particles is satisfactory for this purpose. Preferably the aluminum oxide film is applied to the inner surface of the lamp envelope after the aperture has been formed on the reflector coating but prior to the deposition of the phosphor.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a drawing of an aperture fluorescent lamp having a wider aperture at its ends than at its center.

FIG. 2 is a cross sectional view of the lamp of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT -In one example of an aperture fluorescent lamp in accordance with this invention, lamp envelope 1 was made of glass and was 18 inches long by 1 inch diameter. The inner surface of envelope 1 was coated with an opaque layer 2 of titanium dioxide particles having a mean particle diameter of 0.3 micron in order to produce a reflecting surface. Layer 2 was applied from a suspension of titanium dioxide particles in a nitrocellulose vehicle having butyl acetate as a solvent. The suspension was poured into one end of envelope 1 so as to cover the entire inner surface thereof. After the excess suspension was poured off, the coating was allowed to dry.

tAn aperture 3 was formed on layer 2 by the use of a scraping tool (not shown). The scraping tool was inserted in one end of envelope 1 and was drawn across the inner surface thereof along the entire length, so as to scrape away a portion of layer 2 and leave a clear aperture 3 substantially parallel to the axis of the envelope. Aperture 3 had a width of /2 inch for a distance of 4 /2 inches at each end and a width of /4 inch in the intermediate 9 inch section.

A thin transparent film 4 of aluminum oxide was then applied over the entire inner surface of envelope -1, in order to completely cover reflector layer 2 and aperture 3. Film 4 was substantially opaque to ultraviolet radiation and protected the clear glass at aperture 3 from direct exposure to ultraviolet energy.

Film 4 was applied from a suspension of about 2 grams of ultrafine aluminum oxide particles in milliliters of a liquid vehicle. The aluminum oxide had a mean particle diameter of about 0.01 micron, such a size being small enough to permit the laying down of the desired thin transparent film 4. The liquid vehicle consisted of ethyl cellulose as a binder, xylene and butanol as solvents, dibutyl phthalate as a plasticizer and a small amount of an amine as a dispersing agent. Xylene and butanol were selected as the solvents since they do not dissolve or disrupt the titanium dioxide layer 2.

Envelope 1 was then baked at a temperature of 600 C. for 3 minutes to remove the organic constituents of the coatings and to cause the titanium dioxide and aluminum oxide to adhere to the envelope.

A suspension of a fluorescent phosphor was then applied within envelope 1 to form phosphor coating 5 on film 4. After phosphor coating 5 had dried, the phosphor which covered aperture 3 was scraped off in the same manner as before, leaving only film 4 on the glass thereat. Envelope 1 was then again heated at 600 C. for 3 minutes to remove the organic constituents of the phosphor and to improve the adherence of the phosphor coating. Envelope 1 was then processed into a fluorescent lamp by usual manufacturing techniques.

The light distribution profile of the above lamp was then compared with that of a fluorescent lamp having a uniform aperture, inch wide, for its entire length. Light output was determined by placing an opal diffuser 1% inch from the lamp aperture and measuring the amount of light passing through the diffuser by means of a photometer. The results showed a more uniform light distribution profile from the lamp having a wider aperture at its ends than from the lamp having the prior art aperture. For example, the light intensity of the former lamp at a point four inches from its end was 100% of the intensity at the center while the same measurement was only 90% for the latter. This difference is significant for photocopy purposes since a variation of more than about 5% in the lightdistribution profile results in distortion and illegibilty near the edges of the reproduction.

In another example of an aperture fluorescent lamp having an improved light distribution profile, the angle of the aperture was 60 for 2 /2 inch distance at the ends of the lamp, then 20 for the next 2 /2 inches and finally 35 for the remaining 8 inch length in the center.

Because of the method used in forming the aperture,

where the reflector and phosphor coatings are manually increased, the total light output is also increased. Howwould not increase significantly due to additional loss of phosphor coating is approximately We claim: 1. An aperture fluorescent lamp comprising: an elongated tubular glass envelope having electrodes sealed into its opposite ends and containing an ionizable medium therein; and a coating applied to the interior surface of said glass envelope, said coating comprising a phosphor layer extending over the entire interior of said envelope except for a narrow aperture extending along substantially the whole length of said envelope, said aperture having a width which is greatest towards said envelope ends whereby substantially uniform luminous intensity is achieved along the whole length of said aperture.

References Cited UNITED STATES PATENTS 2,407,379 9/ 1946 Morehouse 313-109 3,094,641 6/1963 Gungle et a1 313-221 X 3,141,990 7/1964 Ray 3l3-l09 X 3,275,872 9/1966 Chernin et a1. 313-4109 3,407,325 10/1968 Brown, Jr 313--10-9 PALMER C. DEMEO, Primary Examiner US. Cl. X.R. 313-413, 220 

